This invention relates to particular zinc borosilicate glass compositions suitable for enamel coatings on soda-lime glass envelopes. More particularly, it relates to said glass compositions which are devoid of lead, arsenic, antimony and cadmium for ecological considerations and produce a reasonably optically clear colored coating when fired with an inorganic color pigment.
Lead-free zinc borosilicate glass compositions are known and have been used as colored frits for bonding to both metal and ceramic substrates. For example, U.S. Pat. No. 3,005,722 discloses such a glass composition for use in ceramic type electroluminescent lamp devices wherein the uncolored frit serves as a binder of phosphor particles when fused to a porcelainized iron plate. Colored enamels using said type glass compositions are also disclosed in U.S. Pat. No. 3,527,649 which can employ various inorganic pigments such as cadmium, selenium, iron, copper, cobalt and other metal compounds to produce highly colored glazes when fired on ceramic ware. A known zinc borosilicate frit glass for adherence to the soda-lime glass bulb surface of an incandescent lamp has a typical analysis in weight percent as follows: ZnO--28.3, SiO.sub.2 --19.4, B.sub.2 O.sub.3 --21.7, Al.sub.2 O.sub.3 --2.7, Na.sub.2 O--4.9, K.sub.2 O--6.1, CaO--4.3, BaO--3.7, TiO.sub.2 --4.5, F--2.8, and Sb.sub.2 O.sub.3 --1.6. This glass composition exhibits a linear thermal coefficient of expansion in the 0.degree.-300.degree. C. temperature range of approximately 70-80.times.10.sup.-7 /C..degree. in order to accommodate the thermal expansion charcteristics of the soda-lime glass when bonded thereto during the firing at elevated temperatures.
In a formerly pending U.S. Pat. Application Ser. No. 148,917, filed May 12, 1980, in the name of W. A. Graff, and assigned to the present assignee, now U.S. Pat. No. 4,280,843, there is disclosed a zinc borosilicate frit enamel which is devoid of both lead and arsenic and which exhibits improved weather durability when fired on incandescent lamp envelopes. This glass frit consists essentially of about in weight percent: 18-25 SiO.sub.2, O-5 TiO.sub.2, 7-12 ZrO.sub.2, 18-21 ZnO, 4-6, BaO, 4-6, CaO, 2-4 Na.sub.2 O, 6-8 K.sub.2 O, 15-22 B.sub.2 O.sub.3, 3-7 Al.sub.2 O.sub.3, and 0-3 F, except for incidental impurities and refining agents. In the preferred embodiments the weight ratio of CaO to BaO is approximately 1 and the aggregate ZnO+B.sub.2 O.sub.3 +SiO.sub.2 weight percent is in the approximate range 58-63%. For other preferred embodiments, the TiO.sub. 2 +ZrO.sub.2 aggregate weight percent is in the approximate range 11-13. The optical opacity of this glass frit, however, does not provide the gloss and clarity usually sought for many indoor lighting applications.
It would still be desirable, therefore, to develop a lead-free frit glass providing a clear or transparent colored appearance together with sufficient chemical durability for indoor lighting applications. It would be further desirable to provide these final product characteristics by compositional modification of the frit glass alone so that customary pigments and subsequent application of the colored enamel to a soda-lime glass envelope need not also be changed.